|M.Sc Student||Gandman Maria|
|Subject||Microstructural Evolution of Al-Cu Intermetallic Phases in|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Wayne D. Kaplan|
|Full Thesis text|
Wire-bonding is the main interconnection process in the packaging industry. Au wires are commonly used for wire-bonding, while Cu wires are a potential replacement for Au due to their superior electrical and mechanical properties. In general, Al-Cu intermetallics have better electrical and thermal qualities than Al-Au intermetallics commonly found at the interface between Au wires and Al contact pads. At the same time, use of Cu wires requires significant changes in the wire-bonding process to prevent oxidation. In order to incorporate Cu in the wire-bonding process, substantial data regarding ageing and intermetallic formation of Al-Cu bonds is required.
In this study as-bonded and heat-treated Al-Cu wire-bonds were investigated. The Al-Cu wire-bonds were heat-treated at 175ºC for 2, 24, 96 and 200 hours in air and in argon. The microstructure of the wire-bonds, and in particular the Al-Cu interface, was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), while the compositional and structural analysis of the Al-Cu interface was conducted by energy dispersive spectroscopy (EDS) by selected area diffraction (SAD) in TEM, respectively.
Discontinuous and non-uniform intermetallic regions were found in the as-bonded and heat-treated samples. The initial intermetallic phase identified was Al2Cu (q), while after longer heat-treatments (96 and 200 hours) Al4Cu9 also formed. Voids and void-lines in the investigated samples were found at the intermetallic-copper or at the intermetallic-intermetallic interfaces. No voids were found at the intermetallic-SiO2 interface. No continuous void-lines were found in specimens heat-treated at 175ºC for up to 200 hours. For specimens heat-treated for 200 hours, creep of the copper wire was detected.
From detailed microstructural analysis, it was found that significant amounts of Al-Cu intermetallics only form in regions subjected to high levels of stress, resulting from loading of the capillary on the Al pad. This is important for optimal design of the capillary shape. Void-lines, which are defects which can lead to failure, were found after long thermal anneals adjacent to the Al4Cu9 phase as a result of volume changes due to formation of Al4Cu9. From analysis of the microstructure, optimal bonding parameters can be derived to increase the life-time of the Al-Cu interconnection.